Use the Pythagorean Theorem to find the length of the side marked "x."
x^2 + (10 mi)^2 = (13 mi)^2. Thus, x^2 + 100 mi^2 = 169 mi^2.
Next, x^2= (169-100) mi^2, or x^2 = 69 mi^2. Find the positive square root of both sides of this equation. What is it?